A shift operation of an automatic transmission of a motor vehicle having an electronic control is generally triggered, at a given road speed of the vehicle, when a load command exceeds or drops below a threshold value. The load command is given by the driver by means of an accelerator pedal.
The engine load, for example, constitutes a load command as described in the publication "Bosch Technische Berichte", Volume 7, (1983), Number 4, pages 160 to 166. The engine load can be determined from the opening of the throttle flap. In these systems, this throttle flap opening is pregiven by an accelerator pedal actuated by the driver. The output rpm of the transmission (transmission output rpm) is used, as a rule, as the read speed of the motor vehicle.
Furthermore, and as described in United States patent application Ser. No. 08/050,085, filed Apr. 28, 1993, now abandoned, an engine torque desired value or an output torque desired value can constitute the load command and is derived from the position of the accelerator pedal.
In transmission control systems, if shifting between the third and fourth gear is considered, when a downshift from fourth gear into third gear takes place when the load command expressed by the driver is greater than an upper threshold value. In contrast, an upshift from third gear into fourth gear takes place when the load command of the driver is less than a lower threshold value. These threshold values are dependent upon the road speed of the vehicle. For a fixed speed, it always applies that the threshold for determining an upshift is less than the threshold for determining a downshift. The differences between these two thresholds for a shifting operation (in this example, between the third and fourth gears) defines the so-called shift hysteresis.
There is a clearly determinable gear which is optimal as to consumption for most operating conditions. A hysteresis in the shift characteristic lines means, however, that this consumption-optimal gear is not always selected. For this reason, it would be purposeful to attempt to achieve a small hysteresis. This, however, would have as a consequence that even very small back and forth movements of the accelerator pedal would cause so-called pendular shifting. If these pendular shifts take place within short time intervals, then these shifts are especially disturbing and act negatively on the wear of the transmission (friction elements defining the clutch) and with respect to driving comfort.